Laser

ABSTRACT

A laser includes a laser medium, a light source for emitting light which pumps the laser medium, a pair of mirrors which are disposed on opposite sides of the laser medium and form a resonator, an etalon which is disposed in the resonator in order to make the oscillation mode of a laser beam a single longitudinal mode and a nonlinear optical crystal which is disposed in the resonator and converts the laser beam to a second harmonic. The effective thickness t of the etalon satisfies the condition that 2t(n SH  -n FM )/λ SH  is substantially an integer wherein λ SH  represents the wavelength of the second harmonic, n FM  represents the refractive index of the etalon for the fundamental wave and n SH  represents the refractive index of the etalon for the second harmonic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a laser, and more particularly to a laserwhich is arranged to output in a single longitudinal mode by an etalondisposed in a resonator.

2. Description of the Related Art

There has been known a solid state laser in which a solid state lasermedium doped with a rare earth metal such as neodymium is pumped with alaser beam emitted from a semiconductor laser (laser diode). In such alaser, it has been put into practice to dispose an etalon in theresonator in order to suppress generation of mode competition noise,thereby making the oscillation mode a single longitudinal mode.

Further, in such a laser, it has been in wide use to dispose a nonlinearoptical crystal in the resonator in order to convert the laser beam toits second harmonic and to obtain a laser beam having a shorterwavelength.

However in a laser in which an etalon and a nonlinear optical crystalare disposed in the resonator, there has been a problem that a secondharmonic reflected at an end face of the etalon appears around the mainbeam of a second harmonic, thereby deteriorating the beam quality of thesecond harmonic. The reason such a problem arises will be described indetail with reference to FIG. 3, hereinbelow.

In FIG. 3, reference numeral 1 denotes a pumping source, referencenumeral 2 denotes a solid state laser medium pumped with pumping light 3emitted from the pumping source 1, reference numeral 4 denotes aresonator mirror, reference numeral 5 denotes a nonlinear opticalcrystal and reference numeral 6 denotes an etalon. The resonator mirror4 and the solid state laser medium 2 form a laser resonator. That is, asurface 4a of the resonator mirror 4 and an end face 2a of the lasermedium 2 are provided with a predetermined coating and these facesfunction as resonator mirrors.

With this arrangement, the laser beam is converted to its secondharmonic by the nonlinear optical crystal 5 and the second harmonicemanates from the resonator in the direction of the optical axis of theresonator mirror 4 as indicated at 7. The second harmonic 7 is the mainbeam.

The properties of the coating on the mirror surface 4a are determined onthe basis of not only the second harmonic but also a fundamental wave sothat the coating functions as a HR (high reflective) coating for thefundamental wave. Due to limitation on coating technique, thereflectance of the mirror surface 4a for the second harmonic isgenerally in the range of about 5 to 10%. That is, it is difficult tomake the mirror surface 4a an antireflective surface (a surface whosereflectance is not higher than 0.5%).

Further since the etalon 6 is disposed generally at an angle not largerthan 5° (preferably 0.3° to 1°) to the optical axis of the resonator,the second harmonic reflected at the mirror surface 4a is reflected atthe etalon 6 at an angle to the main beam 7 and forms a stray beam 7'which emanates from the resonator around the main beam 7.

Further the stray beam 7' reflected at the rear end face of the etalon 6(the end face of the etalon 6 facing the nonlinear optical crystal 5) isreflected again at the front end face of the etalon 6 and furtherreflected at the end face 2a of the laser medium 2 and emanates from theresonator at an angle to the main beam 7 as shown by the chained line.This forms another stray beam 7" which emanates from the resonatoraround the main beam 7.

Since the end face 2a of the laser medium 2 is provided with a HRcoating in order to obtain a high second harmonic output, also the straybeam 7' is well reflected. Even if the end face 2a is provided with anAR coating for the second harmonic, it is difficult to lower thereflectance of the end face 2a to a level not higher than 0.5% for thereason described above in conjunction with the resonator mirror surface4a.

Further since the etalon 6 is generally provided with coating on neitherof the front and rear end faces for the purpose of simplicity,reflection is apt to take place at both the front and rear end faces.Even if an AR coating for the second harmonic is provided on each of thefront and rear end faces of the etalon 6, the second harmonics reflectedat the front and rear end faces interfere with and enhance each othersince the front and rear end faces are plane parallel and the effectivereflectance can sometimes reach about 2%.

SUMMARY OF THE INVENTION

In view of the foregoing observations and description, the primaryobject of the present invention is to prevent, in a laser comprising anetalon and nonlinear optical crystal disposed in a resonator,deterioration in beam quality of the main beam of second harmonic due tothe inclusion of second harmonic reflected at the etalon.

The laser of the present invention comprises an etalon and a nonlinearoptical crystal for generating a second harmonic disposed in a resonatorand is characterized in that the effective thickness t of the etalonsatisfies the condition that 2t(n_(SH) -n_(FM))/λ_(SH) is substantiallyan integer wherein λ_(SH) represents the wavelength of the secondharmonic, n_(FM) represents the refractive index of the etalon for thefundamental wave and n_(SH) represents the refractive index of theetalon for the second harmonic.

The present invention is based on the fact that a fundamental laser beamhaving a wavelength the effective transmittance of the etalon for whichis increased due to interference between the fundamental wave reflectedat the front and rear end faces of the etalon automatically comes tooscillate and on the fact that the wavelength of the second harmonic isstrictly governed by the wavelength of the fundamental wave. That is, inaccordance with the present invention, the thickness of the etalon islimited so that the transmittance of the etalon for the second harmonicis also increased (that is, the reflectance is lowered and the straylight is weakened) due to interference between the second harmonicreflected at the front and rear end faces of the etalon.

This will be described in more detail with reference to FIG. 2,hereinbelow. In FIG. 2, L1 and L2 denote fundamental waves or secondharmonics which have an optical-path difference therebetween andinterfere with each other due to reflection at the front and rear endfaces of the etalon 6.

The laser beam as the fundamental wave oscillates at a wavelength atwhich loss due to interference at the etalon is minimized (thetransmittance is maximized). This state can be formulated as followssince the optical-path difference between L1 and L2 is an integralmultiple of the wavelength of the fundamental wave.

    2tn.sub.FM -2t sin θ1·sin θ2=m.sub.FM (integer)λ.sub.FM                                  ( 1)

wherein n_(FM) represents the refractive index of the etalon for thefundamental wave, θ1 represents the inclination of the etalon(fundamental wave incident angle), θ2 represents the angle of thefundamental wave in the etalon, λ_(FM) represents the wavelength of thefundamental wave and t represents the effective thickness of the etalon.

Further sin θ1=n_(FM) sin θ2 and t cos θ2=t', wherein t' represents thereal thickness of the etalon.

Since the transmittance for the second harmonic is also increased due tointerference at the etalon when the similar condition is satisfied, thefollowing formula (2) can be obtained.

    2tn.sub.SH -2t sin θ1·sin θ2=m.sub.SH (integer)λ.sub.SH                                  ( 2)

wherein n_(SH) represents the refractive index of the etalon for thesecond harmonic, θ1 represents the inclination of the etalon (secondharmonic incident angle), θ2 represents the angle of the second harmonicin the etalon and λ_(SH) represents the wavelength of the secondharmonic.

Since λ_(FM) =2λ_(SH), the left side of formula (2) becomes as follows.

    2t(n.sub.SH -n.sub.FM)+(2tn.sub.FM -2t sin λ1·sin θ2)

    =2t(n.sub.SH -n.sub.FM)+m.sub.FM (integer)λ.sub.FM

    =2t(n.sub.SH -n.sub.FM)+m.sub.FM (integer)2λ.sub.SH

Accordingly when the first term of this formula is an integral multipleof θ_(SH), that is, when

    2t(n.sub.SH -n.sub.FM)=m'.sub.SH (integer)λ.sub.SH

    ∴2t(n.sub.SH -n.sub.FM)/λ.sub.SH =an integer(3),

formula (2) should hold.

As described above, in the laser in accordance with the presentinvention, the effective thickness of the etalon is determined so thatformula (3) should hold. Accordingly, the transmittance of the etalon isincreased for the second harmonic as well as for the fundamental wave,whereby the stray light of second harmonic is reduced and the beamquality can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a solid state laser in accordancewith an embodiment of the present invention,

FIG. 2 is a view for illustrating reflection of light beams at the frontand rear end faces of the etalon, and

FIG. 3 is a schematic view for illustrating generation of stray light inthe conventional lasers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment of the present invention will be described in detail withreference to the drawings, hereinbelow. In all the embodiments describedhereinbelow, the present invention is applied to a laser diode pumpedsolid state laser. In all the embodiments, the arrangement of theelements in the laser diode pumped solid state laser is basically thesame and is as shown in FIG. 1.

In FIG. 1, the laser diode pumped solid state laser comprises asemiconductor laser 11 which emits a laser beam 10 as a pumping beam, acondenser lens 12 which may be, for instance, a rod lens and condensesthe laser beam 10 which is divergent light, a laser crystal doped withneodymium (Nd), a resonator mirror 14 disposed in front of the lasercrystal 13 (on the right side of the laser crystal 13 as seen in FIG.1), a nonlinear optical crystal 15 disposed between the laser crystal 13and the resonator mirror 14, and an etalon 16 disposed between thenonlinear optical crystal 15 and the resonator mirror 14.

These elements are mounted on a common casing (not shown) into a unit.The temperature of the casing is kept at a predetermined temperature bya temperature sensor, a thermoelectric element and a temperaturecontroller (which are not shown) so that the distances between thecomponents and the refractive indexes of the optical elements are notchanged.

In the laser with the arrangement described above, neodymium ionscontained in the laser crystal 13 are excited by the laser beam 10 andemits light. The light resonates between the end faces 13a of the lasercrystal 13 and the mirror surface 14a of the resonator mirror 14 and asolid laser beam 20 is produced. The solid laser beam 20 is converted toits second harmonic 21 having a wavelength equal to 1/2 of the solidlaser beam 20 by the nonlinear optical crystal 15. The second harmonic21 emanates forward from the resonator mirror 14. The oscillation modeof the laser is brought to a single longitudinal mode by the etalon 16,whereby a stabilized output free from mode competition noise can beobtained. The end face 13a of the laser crystal 13 is provided withcoating which is highly reflective for the solid laser beam 20 and thesecond harmonic 21. The mirror surface 14a of the resonator mirror 14 isprovided with coating which is highly reflective for the solid laserbeam 20 and is partly transmissive for the second harmonic 21.

First Embodiment

Nd:YVO₄ crystal, that is, a YVO₄ crystal doped with neodymium (Nd) wasemployed as the laser crystal 13 and was pumped by a laser beam 10 of808 nm, thereby generating a solid laser beam 20 having a wavelength of1064 nm. A LiNbO₃ crystal having periodic domain reversals was employedas the nonlinear optical crystal 15 to convert the solid laser beam 20(as the fundamental wave) to its second harmonic 21 having a wavelength(λ_(SH)) of 532 nm.

Synthetic quartz having plane parallel front and rear faces was employedas the etalon 16. The difference (n_(SH) -n_(FM)) between the refractiveindex n_(SH) of the synthetic quartz etalon for the second harmonic (532nm) and that n_(FM) for the fundamental wave (1064 nm) was about 0.0111.

Since the half-width of the fluorescence spectra of 1064 nm oscillationline of the Nd:YVO₄ is about 0.8 nm, it is preferred that the FSR (FreeSpectral range: longitudinal mode intervals) of the etalon 16 be notsmaller than 0.8 nm in order to make the oscillation mode a singlelongitudinal mode. This condition corresponds to that the thickness ofthe synthetic quartz is not larger than 380 nm.

Taking into account also the requirement on the thickness of the etalon,13 was adopted as the integer in formula (3), that is, 2t(n_(SH)-n_(FM))/λ_(SH) =13, which resulted in the effective thickness t of theetalon 16 of 312 μm (t=312 μm). The etalon 16 was inclined at an anglesmaller than 1° to the optical axis of the resonator. In this particularembodiment, change in the optical path due to the inclination of theetalon was very small and the real thickness t' of the etalon 16 wasmade equal to the effective thickness thereof, that is, t'=t=312 μm.

The etalon 16 was used without coating, reflection of the secondharmonic 21 was weakened for the reason described above and no straylight due to the reflection was observed.

Then the etalon 16 was provided with coating whose reflectance for thesecond harmonic (λ_(SH) =532 nm) was not higher than 0.5%. The realreflectance was 0.2% and 0.5% respectively at the front and rear facesof the etalon 16. The etalon 16 was inserted into the resonator. Also inthis case, reflection of the second harmonic 21 was weakened for thereason described above and only very weak stray light due to thereflection was observed.

Second Embodiment

Nd:YAG crystal, that is, a YAG crystal doped with neodymium (Nd) wasemployed as the laser crystal 13 and was pumped by a laser beam 10 of809 nm, thereby generating a solid laser beam 20 having a wavelength of946 nm. A LiNbO₃ crystal having periodic domain reversals was employedas the nonlinear optical crystal 15 to convert the solid laser beam 20(as the fundamental wave) to its second harmonic 21 having a wavelength(λ_(SH)) of 473 nm.

Calcite having plane parallel front and rear faces was employed as theetalon 16. The difference (n_(SH) -n_(FM)) between the refractive indexn_(SH) (refractive index for light polarized in the direction of a-axis)of the calcite etalon for the second harmonic (473 nm) and that n_(FM)for the fundamental wave (946 nm) was about 0.0252.

Taking into account the value of the difference (n_(SH) -n_(FM)), 35 wasadopted as the integer in formula (3), that is, 2t(n_(SH)-n_(FM))/λ_(SH) =35, which resulted in the effective thickness t of theetalon 16 of 328 μm (t=328 μm). The etalon 16 was inclined at an anglesmaller than 1° to the optical axis of the resonator. In this particularembodiment, change in the optical path due to the inclination of theetalon was very small and the real thickness t' of the etalon 16 wasmade equal to the effective thickness thereof, that is, t'=t=328 μm.

The etalon 16 was provided with coating whose reflectance for the secondharmonic (λ_(SH) =473 nm) was not higher than 0.5%. The real reflectancewas 0.2% and 0.4% respectively at the front and rear faces of the etalon16. The etalon 16 was inserted into the resonator. Also in this case,reflection of the second harmonic 21 was weakened for the reasondescribed above and only very weak stray light due to the reflection wasobserved.

Third Embodiment

Nd:YLF crystal, that is, a YLF crystal doped with neodymium (Nd) wasemployed as the laser crystal 13 and was pumped by a laser beam 10 of795 nm, thereby generating a solid laser beam 20 having a wavelength of1313 nm. A LiNbO₃ crystal having periodic domain reversals was employedas the nonlinear optical crystal 15 to convert the solid laser beam 20(as the fundamental wave) to its second harmonic 21 having a wavelength(λ_(SH)) of 657 nm.

Synthetic quartz having plane parallel front and rear faces was employedas the etalon 16. The difference (n_(SH) -n_(FM)) between the refractiveindex n_(SH) of the synthetic quartz etalon for the second harmonic (657nm) and that n_(FM) for the fundamental wave (1313 nm) was about 0.0096.

Taking into account the value of the difference (n_(SH) -n_(FM)), 10 wasadopted as the integer in formula (3), that is, 2t(n_(SH)-n_(FM))/λ_(SH) =10, which resulted in the effective thickness t of theetalon 16 of 342 μm (t=342 μm). The etalon 16 was inclined at an anglesmaller than 1° to the optical axis of the resonator. In this particularembodiment, change in the optical path due to the inclination of theetalon was very small and the real thickness t' of the etalon 16 wasmade equal to the effective thickness thereof, that is, t'=t=342 μm.

The etalon 16 was provided with coating whose reflectance for the secondharmonic (λ_(SH) =657 nm) was not higher than 0.5%. The real reflectancewas 0.4% and 0.5% respectively at the front and rear faces of the etalon16. The etalon 16 was inserted into the resonator. Also in this case,reflection of the second harmonic 21 was weakened for the reasondescribed above and only very weak stray light due to the reflection wasobserved.

Fourth Embodiment

Nd:YVO₄ crystal was employed as the laser crystal 13 and was pumped by alaser beam 10 of 808 nm, thereby generating a solid laser beam 20 havinga wavelength of 1064 nm. A LiNbO₃ crystal having periodic domainreversals was employed as the nonlinear optical crystal 15 to convertthe solid laser beam 20 (as the fundamental wave) to its second harmonic21 having a wavelength (λ_(SH)) of 532 nm.

Synthetic quartz having plane parallel front and rear faces was employedas the etalon 16. The difference (n_(SH) -n_(FM)) between the refractiveindex n_(SH) of the synthetic quartz etalon for the second harmonic (532nm) and that n_(FM) for the fundamental wave (1064 nm) was about 0.0111.

The etalon 16 was inclined at 1° to the optical axis of the resonator.In this case, since the refractive index n_(SH) of the synthetic quartzetalon for the second harmonic (532 nm) is 1.4607, the ratio of the realthickness t' of the etalon 16 to the effective thickness t (t'/t) is0.999927.

The thickness of the synthetic quartz etalon should be not larger than380 nm in view of FSR of the etalon 16 as in the first embodiment.

Taking into account also the requirement on the thickness of the etalon,14 was adopted as the integer in formula (3), that is, 2t(n_(SH)-n_(FM))/λ_(SH) =14, which resulted in the effective thickness t of theetalon 16 of 335.50 um (t=335.50 μm). Theoretically the real thicknessof the etalon 16 should have been 335.48 μm (335.50×0.999927). Howeveran etalon 16 335 μm thick was used.

The etalon 16 was provided with coating whose reflectance for the secondharmonic (λ_(SH) =532 nm) was not higher than 0.5%. The real reflectancewas 0.2% and 0.5% respectively at the front and rear faces of the etalon16. The etalon 16 was inserted into the resonator. Also in this case,reflection of the second harmonic 21 was weakened for the reasondescribed above and only very weak stray light due to the reflection wasobserved.

Fifth Embodiment

Nd:YVO₄ crystal was employed as the laser crystal 13 and was pumped by alaser beam 10 of 808 nm, thereby generating a solid laser beam 20 havinga wavelength of 1064 nm. A LiNbO₃ crystal having periodic domainreversals was employed as the nonlinear optical crystal 15 to convertthe solid laser beam 20 (as the fundamental wave) to its second harmonic21 having a wavelength (λ_(SH)) of 532 nm.

Synthetic quartz having plane parallel front and rear faces was employedas the etalon 16. The difference (n_(SH) -n_(FM)) between the refractiveindex n_(SH) of the synthetic quartz etalon for the second harmonic (532nm) and that n_(FM) for the fundamental wave (1064 nm) was about 0.0111.

The thickness of the synthetic quartz etalon should be not larger than380 nm in view of FSR of the etalon 16 as in the first embodiment.

Taking into account also the requirement on the thickness of the etalon,14 was adopted as the integer in formula (3), that is, 2t(n_(SH)-n_(FM))/λ_(SH) =14, which resulted in the effective thickness t of theetalon 16 of 335 μm (t=335 μm). The etalon 16 was inclined at an anglesmaller than 1° to the optical axis of the resonator. In this particularembodiment, change in the optical path due to the inclination of theetalon was very small and the real thickness t' of the etalon 16 mightbe equal to the effective thickness thereof, that is, t'=t=335 μm.However an etalon 16 which was 339 μm in thickness was used. That is,t'=t=339 μm.

In this case, the value of 2t (n_(SH) -n_(FM))/λ_(SH) =14.15 and wasslightly deviated from the optimal condition.

The etalon 16 was used without coating. Reflection of the secondharmonic 21 was weakened for the reason described above and stray lightdue to the reflection was very weak though the effective thickness ofthe etalon 16 was slightly deviated from the optimal condition.

Sixth Embodiment Nd:YVO4 crystal was employed as the laser crystal 13and was pumped by a laser beam 10 of 808 nm, thereby generating a solidlaser beam 20 having a wavelength of 1064 nm. A LiNbO₃ crystal havingperiodic domain reversals was employed as the nonlinear optical crystal15 to convert the solid laser beam 20 (as the fundamental wave) to itssecond harmonic 21 having a wavelength (λ_(SH)) of 532 nm.

Synthetic quartz having plane parallel front and rear faces was employedas the etalon 16. The difference (n_(SH) -n_(FM)) between the refractiveindex n_(SH) of the synthetic quartz etalon for the second harmonic (532nm) and that n_(FM) for the fundamental wave (1064 nm) was about 0.0111.

The thickness of the synthetic quartz etalon should be not larger than380 nm in view of FSR of the etalon 16 as in the first embodiment.

Taking into account also the requirement on the thickness of the etalon,14 was adopted as the integer in formula (3), that is, 2t(_(SH)-n_(FM))/λ_(SH) =14, which resulted in the effective thickness t of theetalon 16 of 335 μm (t=335 μm). The etalon 16 was inclined at an anglesmaller than 1° to the optical axis of the resonator. In this particularembodiment, change in the optical path due to the inclination of theetalon was very small and the real thickness t' of the etalon 16 mightbe equal to the effective thickness thereof, that is, t'=t=335 μm.However an etalon 16 which was 337 μm in thickness was used. That is,t'=t=337 μm.

In this case, the value of 2t(n_(SH) -n_(FM))/λ_(SH) =14.06 and wasslightly deviated from the optimal condition.

The etalon 16 was provided with coating whose reflectance for the secondharmonic (λ_(SH) =532 nm) was not higher than 0.5%. The real reflectancewas 0.1% and 0.5% respectively at the front and rear faces of the etalon16.

Also in this case, reflection of the second harmonic 21 was weakened forthe reason described above and stray light due to the reflection wasvery weak though the effective thickness of the etalon 16 was slightlydeviated from the optimal condition. The second harmonic 21 emanatingfrom the resonator mirror 14 was practical in beam quality.

The present invention can also be applied to various solid state lasersother than those described above.

What is claimed is:
 1. A laser comprising a laser medium, a light sourcefor emitting light which pumps the laser medium, a pair of mirrors whichare disposed on opposite sides of the laser medium and form a resonator,an etalon which is disposed in the resonator in order to make theoscillation mode of a laser beam a single longitudinal mode and anonlinear optical crystal which is disposed in the resonator andconverts the laser beam to a second harmonic, wherein the improvementcomprises thatthe effective thickness t of the etalon satisfies thecondition that 2t(n_(SH) -n_(FM))/λ_(FM) is substantially an integerwherein λ_(SH) represents the wavelength of the second harmonic, n_(FM)represents the refractive index of the etalon for the fundamental waveand n_(SH) represents the refractive index of the etalon for the secondharmonic.